Successful waterborne acrylic (or latex emulsion) coating compositions must deliver performance comparable to traditional solvent-borne coatings, while meeting increasingly stringent VOC (volatile organic compounds) emission regulations. It is known in the coatings art that the formation of a latex film can be facilitated by incorporating a coalescing agent in the latex emulsion. These coalescing agents, or coalescents, facilitate the individual latex particles coming together to thereby form a continuous film at a given temperature by reducing the minimum film-forming temperature (MFFT) of the latex polymer. Thus, as used throughout, the coalescents of the invention reduce the minimum film-forming temperatures of the coating compositions in which they are used, as further described below.
Traditional coalescents are described as fugitive coalescents since they tend to escape from the coating film over time. As a result, in some jurisdictions, such coalescents are regulated as VOCs, or volatile organic compounds. A need therefore exists for a non-fugitive coalescent that is capable of meeting the VOC regulations imposed on the paint industry.
Additionally, in recent years, there has been increased interest in renewable, bio-based materials, also due to environmental concerns. It is therefore an object of the invention to provide coalescents that may be used in coating compositions that are low in VOC emissions, and that can be made from bio-based materials.
Levulinic acid, or 4-ketopentanoic acid, is a bio-based chemical that can be produced with low cost by a biorefinery process using low-grade cellulosic by-products from paper mills and other biomass sources. Alternatively, levulinic acid may be prepared, for example, by heating sucrose with concentrated hydrochloric acid, or by other conventional synthetic methods. It may also be prepared by acid-catalyzed hydrolysis of cellulose. The generally accepted reaction pathway is, in a first step, to break down the polymer chains of cellulose into low molecular weight fragments, and eventually to glucose, which further decomposes to 5-hydroxymethylfurfural (HMF). In a second step, HMF is converted to levulinic acid and formic acid.
Levulinic ester derivatives are known plasticizers for cellulosic materials in organic solvent-based systems. Thus, U.S. Pat. No. 2,004,115 discloses the preparation of levulinic esters of modified polyhydric alcohols and their use as plasticizers for cellulose derivatives. U.S. Pat. No. 2,008,720 discloses the preparation of levulinic esters of monohydric alcohols and their applications as plasticizers for cellulose derivatives.
WO 2007/094922 discloses ester derivatives of levulinic acid that are useful as plasticizers and/or coalescent solvents in polymer compositions. The ester derivatives disclosed are the reaction products of levulinic acid with a polyol, for example glycerol. The polyols referred to are polyhydric alcohols having two or more hydroxyl groups, for example a sugar alcohol such as sorbitol, mannitol, erythritol, lactitol, maltitol, xylitol, glycerol, and the like.
U.S. Pat. Appln. Publn. No. 2008/0242721 discloses the preparation of ketal compounds from glycerol and levulinic acid, and the use of such ketal compounds as plasticizers.
EP 0501614 discloses the use of esters of keto-carboxylic acids as coalescing aids in paints and coatings. The document discloses that the coalescing aid should be volatile enough to escape when the applied paint is allowed to dry but should evaporate more slowly than water, which is the liquid carrier for the latex. N-butyl esters of the keto-carboxylic acids, especially 4-oxo-valeric acid and 5-oxo-caproic acid, are said to be most preferred. In this disclosure, the chain length of the alcohols used in the composition is from 2 to 8 carbon atoms.
While those skilled in the art would expect esters formed from an alcohol with a carbon chain length longer than eight carbons to be too hydrophobic and too bulky to be useful as coalescents, the present inventors have surprisingly discovered that the levulinic acid esters of a monohydric alcohol having a carbon chain length of at least 10 carbon atoms are effective coalescents, providing latex coating compositions having a reduced VOC content compared with those taught in the art, and thus contrary to the teaching that coalescing aids should readily evaporate. The levulinic acid esters according to the invention also exhibit improved coalescing activity compared with esters of polyols such as glycerol, as further elaborated below.